The 4th paroxysm within little more than 4 days is occurring at the time of this update! Starting in the evening of 20 Feb 2021, the explosive activity at the New SE crater gradually increased and developed into sustained lava fountains now approx. 300 m tall, and still rising.
A lava flow has started to descend towards the Valle del Bove and although obscured by the night, a tall ash and steam plume is rising above the summit region of the volcano. As spectacular as it looks, these eruptions pose no threat to inhabited areas, although the ash plumes can be dangerous to aircraft and the lapilli and ash falls are a (rather common) nuisance to the downwind areas around the volcano.
It is remarkable to see that tonight's paroxysm came at almost exactly the same interval as the intervals between the preceding paroxysms on 16 Feb evening, 18 Feb early morning, and 19 Feb late morning, i.e. each approx. 35 hours after the previous one.
What causes such regular eruptions?
The current, remarkably cyclic occurrence of these short, but very violent lava fountain phases at near-perfectly regular intervals is certainly an extraordinary and rare event, but has been observed at Etna several times in the past, such as in 2000 and 2012-13. Intervals between paroxysms ranging from 12 hours or less to several days.
What causes this regularity exactly is not fully understood, but the cyclic behavior at the surface is certainly linked to a combination of processes deeper below, which govern the rise of fresh, gas-rich magma from deeper reservoirs, capable to produce fountains when erupted at the surface. This hotter magma obviously rises in separate batches, like large bubbles from a leak, at regular intervals as opposed to a more or less constant flow of magma.The latter, however, is a more common type of behavior, resulting in prolonged phases of activity at moderate levels, much as had been observed for weeks and months in a row during the past few years.
This difference is also reflected by the tremor amplitude: while extreme during the short paroxysms, it has been much lower during the intervals as during the weeks before, where activity fluctuated around an intermediate average value.
In other words, right now, Etna seems to be in a different state of transporting magma from its reservoirs to the surface, one characterized by generally very low rates of flux interrupted by short, extreme bursts of activity when individual batches make their way upwards.
How long the current pattern will last and where it might lead to, can only be guessed.